Transducers convert signals from one domain to another and are often an integral component in sensors. One common sensor including a transducer that is seen in everyday life is a microphone that converts sound waves to electrical signals.
Microelectromechanical system (MEMS) based sensors include a family of transducers produced using micromachining techniques. MEMS, such as a MEMS microphone, gather information from the environment by measuring the change of physical state in the transducer and transferring a transduced signal to processing electronics that are connected to the MEMS sensor. MEMS devices may be manufactured using micromachining fabrication techniques similar to those used for integrated circuits.
MEMS devices may be designed to function as, for example, oscillators, resonators, accelerometers, gyroscopes, pressure sensors, microphones, and micro-mirrors. Many MEMS devices use capacitive sensing techniques for transducing the physical phenomenon into electrical signals. In such applications, the capacitance change in the sensor is converted to a voltage signal using interface circuits.
One such capacitive sensing device is the MEMS microphone. A MEMS microphone generally has a deflectable membrane separated by a small distance from a rigid backplate. In response to a sound pressure wave incident on the membrane, the membrane deflects towards or away from the backplate, thereby changing the separation distance between the membrane and backplate. Generally, the membrane and backplate are made out of conductive materials and form “plates” of a capacitor. Thus, as the distance separating the membrane and backplate changes in response to the incident sound wave, the capacitance changes between the “plate” and an electrical signal is generated.
For capacitive MEMS sensors, it is possible, in the presence of a large physical signal or shock, for one of the deflectable plates to deflect until contacting an adjacent plate. In such cases, the voltage applied to the plates may be sufficient to cause the plates to remain in contact with one another. This phenomenon may be referred to as “pull-in.” In capacitive MEMS sensors, pull-in may affect the performance of the sensor.